Method for producing bottle-shaped container mainly made of polyethylene terephthalate resin

ABSTRACT

A method for producing a bottle-shaped container that is highly thermally resistant and shows an excellent shaping/shape-keeping property in a short period of time is provided. The method for producing the bottle-shaped container mainly made of polyethylene terephthalate comprises the steps of: first biaxially-oriented blow-molding a preform mainly made of polyethylene terephthalate in a first metal mold, to form a primary intermediate molded article; causing thermal contraction of the primary intermediate molded aritcle by heating the primary intermediate molded article, to form a secondary intermediate molded article; and second blow-molding the secondary intermediate molded article in a second metal mold, to form the bottle-shaped container, characterized in that in the second biaxially-oriented blow-molding step, fluid is blown into the secondary intermediate molded article to expand the secondary intermediate article, and then further fluid is blown into and circulated in the expanded article to cool the expanded article.

BACKGROUND OF THE INVENTION

This invention relates to a method for producing a bottle mainly made ofpolyethylene terephthalate (to be referred to as PET hereinafter). Moreparticularly, it relates to a method for producing a bottle mainly madeof polyethylene terephthalate that is highly thermally resistant andshows an excellent shaping/shape-keeping property.

PET is being popularly and broadly used for containers for beverageetc., because of its high transparency and mechanical strength. PETcontainers are normally produced by biaxially-oriented blow-molding.

Biaxially-oriented blow-molding is popularly used for molding beveragebottles and other containers mainly made of PET. When producing a PETcontainer by this method, pressurized fluid, which is typically air, isblown into a preform that has been prepared in advance by injectionmolding.

In the case of beverage bottles and other containers for containingfood, the content filled in the bottle may be heated for sterilizationand/or other purposes, so that containers are required to be thermallyresistant. To produce a thermally resistant PET container bybiaxially-oriented blow-molding, the metal mold is heated to arelatively high temperature between 140 and 150° C., and the moldedbottle is heat set in order to raise the thermal resistance of themolded container or bottle.

However, the above described biaxially-oriented blow-molding process maygive rise to a shape-keeping problem to the molded container. If theblow-molding process is conducted with a metal mold heated to atemperature level higher than the glass transition temperature (Tg), themolded bottle may produce sinks as the metal mold temperature rises, sothat the capacity or the volume of the molded bottle falls or decreaseslinearly. Particularly, if the temperature of the metal mold is above110° C., the appearance of the bottle may be remarkably damaged. Thus,there is a demand for a method for producing a bottle that is highlythermally resistant and shows an excellent shaping/shape-keepingproperty.

Additionally, since bottles mainly made of PET are normally produced ona mass production basis, there is a demand for reducing the producingtime.

SUMMARY OF THE INVENTION

Therefore, it is the object of the present invention to provide a methodfor producing a container, or a bottle, that is highly thermallyresistant and shows an excellent shaping/shape-keeping property in ashort period of time.

In an aspect of the present invention, the above object is achieved byproviding a method for producing a bottle mainly by using PET comprisingthe steps of:

forming a primary intermediate molded article by primarybiaxially-oriented blow-molding, using a preform mainly made of PET anda first metal mold;

forming a secondary intermediate molded article by heating to causethermal contraction of the primary intermediate molded article; and

subjecting the secondary intermediate molded article to secondaryblow-molding, using a second metal mold;

the secondary intermediate molded article being expanded by blowingfluid into it and subsequently cooled by causing the fluid to circulatein it in the step of secondary blow-molding.

Preferably, fluid under high pressure is blown into the secondaryintermediate molded article for a high pressure blow time of maintaininghigh pressure to reduce the high pressure blow time.

Preferably, said primary biaxially-oriented blow-molding is realized byblowing fluid under high pressure into the preform for the high pressureblow period of maintaining high pressure to reduce the high pressureblow time.

Preferably, said high pressure blow time is a period of time suitablefor providing a good shaping/shape-keeping property.

PREFERRED EMBODIMENTS OF THE INVENTION

According to the invention, firstly a preform is prepared by injectionmolding of the raw material, which is mainly made of PET. The preformmay be prepared by any known method for the purpose of the invention.

For the purpose of the invention, a raw material that is mainly PET maybe resin comprising only PET, or resin added or blended with a barriersubstance and/or other additives to polyethylene terephthalate. Barriersubstances that can be used for the purpose of the inventionnon-limitatively include polyethylene naphthalate (PEN), methaxylylenediamineadipamide (MXD-6) and ethylene-vinylacetate copolymer (EVOH),although some other known barrier substance may alternatively be used.

Both the raw material and the bottle may have a single-layer structureor a multi-layer structure. For example, a three-layer or five-layerstructure formed by using two or more resins selected from PET, abarrier substance, a blend thereof and other substances may be used,although some other structure may alternatively be used.

Then, said preform is biaxially-oriented blow-molded to form a primaryintermediary molded article (first biaxially-oriented blow-molding). Aknown biaxially-oriented blow-molding method may be used for the firstbiaxially-oriented blow-molding. The preform is heated to temperature(90 to 130° C.) that allows a blow-molding operation to be conducted,and fluid (such as air) under high pressure is blown into the perform,using a first metal mold. The temperature of the first metal mold ispreferably between 50 and 230° C.

The known biaxially-oriented blow-molding method may be used for thefirst blow-molding as described above. In addition, according to thepresent invention, the high pressure blow time (during which highpressure is maintained in the first biaxially-oriented blow-molding) canbe reduced, because the shaping/shape-keeping property of the finalproduct is improved by conducting an operation of circulating air in thesecond blow-molding step in a manner as described hereinafter and hencethe primary intermediate molded article can be prepared even if thefluid under high pressure is blown in a short period of time.

The primary intermediate molded article is made larger than the finalproduct of bottle, and subsequently turned into a secondary intermediatemolded article.

The primary intermediate molded article obtained by the firstbiaxially-oriented blow-molding step is released from the first metalmold, and is heated to 110 to 255° C. to force the primary intermediatemolded article to cause thermal contraction of the primary intermediateproduct to form a secondary intermediate molded article. The heatingtemperature is preferably higher than the temperature of the first metalmold used for the first biaxially-oriented blow-molding step by 20 to60° C. As the primary intermediate molded article is heated to such hightemperature, the internal stress of the molded product generated in thefirst biaxially-oriented blow-molding step is alleviated, and theprimary intermediate molded article is allowed to thermally contract toturn itself into a secondary intermediate molded article.

The secondary intermediate molded article has dimensions substantiallysame as or slightly smaller than the final product of the container.

The secondary intermediate molded article obtained as a result ofthermal contraction is subjected to a secondary blow-molding step, usinga second metal mold heated to 60 to 120° C., to obtain a final productof the container.

In the secondary blow-molding step, air is forced to circulate in themolded product at room temperature in the present invention. Morespecifically, fluid (air) under high pressure is supplied through theinside of a drawing core shaft to flow into the secondary intermediaryblow molded product to produce a final product of bottle as theconventional method. According to the present invention, after theintermediate molded article is expanded by blowing the fluid under highpressure, further fluid such as air is continuously blown into theintermediate molded article, and is forced to circulate in and cool themolded article.

The pressure level required for the air circulation may be slightlylower than the pressure level required for the blow-molding, because itis only necessary to force air to circulate in the molded article. Thepressure for the air circulation may be lowered during the aircirculation gradually or stepwise. If the air pressure is reducedstepwise, it may be reduced in a single step or in two steps.

The duration of the secondary blow-molding step includes (1) the timenecessary for raising the fluid (or air) pressure to a predeterminedpressure level for blow-molding (e. g., about 40 MPa), (2) the timenecessary for blowing fluid under sustained high pressure forblow-molding, and (3) the time necessary for reducing the pressure andcausing fluid to circulate in the molded article. When the pressure islowered stepwise (in a single step), the duration includes (1) the timenecessary for raising the pressure to a predetermined pressure level,(2) the time necessary for blowing fluid under sustained high pressurefor blow-molding, and (3) the time necessary for reducing the pressureto an intermediate pressure level (e. g., about 10 to 20 MPa) andcausing air to circulate in the molded product (from the time of thestart of lowering the pressure level to the time when the sustenance ofthe intermediary pressure level is terminated).

According to the invention, although the time (3) for air circulation isfurther required if compared with a conventional process (having no aircirculation), the time (2) for blowing fluid under high pressure can bereduced according to the present invention. Thus, according to thepresent invention, it is possible to reduce the total time necessary forthe second blow-molding step. Note that the time (2) necessary forblowing air under high pressure is such that an excellentshaping/shape-keeping property is appropriately obtained for the finalproduct.

The temperature of the second metal mold in the present invnetion doesnot need to be raised to the high temperature level (140 to 150° C.)like the above described conventional methods in which abiaxially-oriented blow-molding operation is carried out in a singlestep. In other words, for the purpose of the present invention, thetemperature of the second metal mold may be as low as about 105° C. forproducing a thermally resistant bottle.

Otherwise, any known conventional blow-molding method may be applied tothe second blow-molding step of the method according to the invention.

As described above, according to the invention, it is now possible toproduce a container mainly made of PET having highly thermally resistantand showing an excellent shaping/shape-keeping property in a relativelyshort period of time if compared with the prior art. Presumably, this isbecause, as fluid (air) is forced to circulate in the secondblow-molding step, a convection current of air arises in the moldedarticle in the second metal mold, to cool and harden the inner surfaceof the molded article so that, as a result, the molded product shows anexcellent shaping/shape-keeping property.

Since the molded product shows an excellent shaping/shape-keepingproperty, the time necessary for blowing fluid (air) under high pressurecan be reduced to make it possible to reduce the total time necessaryfor the second blow-molding step and improve the production efficiency.

Additionally, since the primary intermediate molded article is heated inthe heating/contraction step to raise the crystal density and to reducethe internal stress, the molded product is provided with a high thermalresistance. Therefore, a highly thermally resistant container can beproduced even if the temperature of the second metal mold used in thesecond blow-molding step is not raised remarkably. Thus, the timerequired for raising the temperature of the second metal mold to apredetermined temperature level is reduced along with the time necessaryfor blowing air under high pressure, to make it possible to produce abottle that is highly thermally resistant and shows an excellentshaping/shape-keeping property in a reduced period of time.

The method for the present invention is applicable not only to producingbeverage bottles but also to producing other containers.

EXAMPLES

In each example and comparative example, 10 bottles were molded asspecimens.

Comparative Example 1

Ten performs were prepared by injection molding of PET.

Each of the performs was heated to 110° C. and is biaxially-orientedblow-molded in the first metal mold, to obtain a primary intermediatemolded article. The temperature of the first metal mold was 160° C., andthe air blowing time was 2.95 seconds as listed in Table 1.

The primary intermediate molded article was taken out of the first metalmold, and heated to 185° C. to be thermally contracted to obtain asecondary intermediate molded article.

The secondary intermediate molded article was heated to 160° C., andblow-molded in a second metal mold, to obtain a final product of bottle.The temperature of the second metal mold was 105° C., the temperature ofthe air was 25° C., and the air blowing time under high pressure (39MPa) was 2.85 seconds as listed in Table 1. Further air was not blown tocirculate in the secondary blow-molding step in this comparativeexample.

The obtained 10 final products were observed for total height andcapacity, and the averages were calculated. Similarly, theshaping/shape-keeping property of each of the specimens was checked bydetermining the standard deviation of the total height and that of thecapacity of the specimen. The obtained results are summarily shown inTable 1.

The thermal resistance of each of the obtained final products was testedby filling the product with hot water heated to 93° C., hermeticallysealing the bottle, showering it with hot water heated to 75° C. for 3minutes, and by visually observing it for deformation. Table 1 alsoshows the obtained results.

EXAMPLE 1

The process of Comparative Example 1 was followed except that the timeof blowing air under high pressure was reduced in the secondblow-molding step and that air was forced to circulate in theintermediate molded article in the secondary blow-molding step. 10specimens of final products were obtained as in Comparative Example 1.

Air was made to circulate, while reducing the air pressure from a highpressure level (39 MPa) to 25.3 Mpa, maintaining 25.3 Mpa (for 0.2seconds from the pressure fall to the termination of sustenance ofpressure), further reducing the air pressure from 23.5 MPa to 16.6 MPaand maintaining 16.6 MPa (for 0.1 seconds from the pressure fall to thetermination of the sustenance of pressure), and subsequently releasedthe air. Thus, the total air circulation time was 0.3 seconds.

The time of blowing air under high pressure in the second blow-moldingstep, the air circulation time in the second blow-molding step, theshaping/shape-keeping property, and the thermal resistance of the finalproducts are listed in Table 1.

EXAMPLES 2 through 4

In each example, the process of Example 1 was followed except that thetime of blowing air under high pressure in the second blow-molding stepwas modified and the blowing time of the first biaxially-orientedblow-molding step was reduced. 10 specimens of final products wereprepared in each example.

The blowing time of the first blow-molding step, the time of blowing airunder high pressure in the second blow-molding step, and the aircirculation time in the second blow-molding step, theshaping/shape-keeping property and the thermal resistance of the finalproducts are listed in Table 1.

Note that the time of blowing air under high pressure of Example 4 maynot be suitable for providing a good shaping/shape-keeping property.Therefore, as far as the time of blowing air under high pressure,Example 4 should be regarded as comparative example.

TABLE 1 Shaping/ Shaping/ shape- shape- Blow Blow time in keepingkeeping time second property property in blow-moldin (total (capacity orfirst under height) volume) blow- high air standard standard Thermalmolding pressure circulation total average deviation average deviationresistance Com. 2.95 sec. 2.85 sec. none 2.85 sec. 253.18 mm 0.20 914.09ml 3.46 no deformation Ex. 1 Ex. 1 2.95 sec. 2.55 sec. 0.3 sec. 2.85sec. 253.14 mm 0.12 924.82 ml 1.09 no deformation Ex. 2 2.20 sec. 1.80sec. 0.3 sec. 2.10 sec. 253.07 mm 0.07 921.16 ml 1.33 no deformation(−0.75 sec.) (−0.75 sec.) Ex. 3 1.70 sec. 1.30 sec. 0.3 sec. 1.60 sec.253.07 mm 0.09 918.58 ml 1.53 no deformation (−1.25 sec.) (−1.25 sec.)Ex. 4 1.45 sec. 1.05 sec. 0.3 sec. 1.35 sec. 253.01 mm 0.07 919.59 ml2.03 no deformation (−1.50 sec.) (−1.50 sec.)

By comparing Comparative Example 1 and Example 1, it will be seen thatthe standard deviation of total height is reduced and the standarddeviation of capacity is remarkably improved, if air is circulatedwithout changing the total blow time in the secondary blow-molding stepeven if the high pressure blow time of the second blow-molding step isreduced. Additionally, the thermal resistance was satisfactory.

When the high pressure blow time of the second blow-molding step isfurther reduced along with the high pressure blow time of the firstbiaxially-oriented blow-molding step, the shaping/shape-keeping propertyis improved particularly in terms of total height as seen from theresults of Examples 2 and 3. When the parameters of Example 3 are used,the blow time of the first blow-molding step is reduced from 2.20seconds of Comparatively Example 1 to 1.70 seconds, and the blow time ofthe second blow-molding step is reduced from 2.10 seconds of theComparatively Example 1 to 1.60 seconds, to realize an overall timereduction of about 30%.

The shaping/shape-keeping property is also improved when the parametersof Example 4 are used.

As described above in detail, according to the invention, air is made toflow in the molded bottle by forcing air to circulate in the secondmolding step, to cool the molded bottle from the inside so as tosuppress any excessive crystal growth. As a result, theshaping/shape-keeping property of the bottle is improved so that thedimensional variances of products are reduced and the ratio of defectiveproducts is minimized to improve the cost performance of bottleproducing.

Additionally, the high pressure blow time is reduced to by turn reducethe cycle time of the process of molding a perform and producing abottle as final product and hence improve the productivity.

1. A method for producing a bottle-shaped container mainly made ofpolyethylene terephthalate, comprising the steps of: firstbiaxially-oriented blow-molding a preform mainly made of polyethyleneterephthalate in a first metal mold, to form a primary intermediatemolded article, causing thermal contraction of the primary intermediatemolded article by heating the primary intermediate molded article, toform a secondary intermediate molded article, and second blow-moldingthe secondary intermediate molded article in a second metal mold, toform the bottle-shaped container, characterized in that in the secondbiaxially-oriented blow-molding step, fluid under high pressure is blowninto the secondary intermediate molded article to expand the secondaryintermediate article, and then further fluid is blown into andcirculated in the expanded article to cool the expanded article, so asto minimize the high pressure blow time.
 2. The method according toclaim 1, wherein said first biaxially-oriented blow-molding is realizedby blowing fluid under high pressure into the preform so as to minimizethe high pressure blow time.
 3. The method according to claim 1, whereinsaid high pressure blow time is a period of time suitable for providinga good shaping/shape-keeping property.
 4. The method according to claim1, wherein said high pressure blow time is a period of time suitable forproviding a good shaping/shape-keeping property.
 5. The method accordingto claim 1, wherein high pressure is greater than 20 Mpa.
 6. The methodaccording to claim 1, wherein high pressure is greater than 25.3 Mpa. 7.A method for producing a bottle-shaped container mainly made ofpolyethylene terephthalate, comprising the steps of: firstbiaxially-oriented blow-molding a preform mainly made of polyethyleneterephthalate in a first metal mold, to form a primary intermediatemolded article, causing thermal contraction of the primary intermediatemolded article by heating the primary intermediate molded article, toform a secondary intermediate molded article, and second blow-moldingthe secondary intermediate molded article in a second metal mold, toform the bottle-shaped container, characterized in that in the secondbiaxially-oriented blow-molding step, fluid is blown into the secondaryintermediate molded article to expand the secondary intermediatearticle, and further fluid is blown into and circulated in the expandedarticle; wherein said first biaxially-oriented blow-molding is realizedby blowing fluid under high pressure into the preform so as to minimizethe high pressure blow time.
 8. The method according to claim 7, whereinsaid high pressure blow time is a period of time suitable for providinga good shaping/shape-keeping property.
 9. The method according to claim7, wherein high pressure is greater than 20 MPa.
 10. The methodaccording to claim 1, wherein high pressure is greater than 25.3 MPa.